Raman scattering is an inelastic scattering of a photon which creates or annihilates an optical phonon. Raman scattering is the result of the interaction of incident photons with chemical molecular vibrations (phonons). A unique chemical molecular structure results in a unique Raman scattering spectrum. Therefore, Raman scattering provides spectral fingerprint details about the chemicals, and can also be used to distinguish molecular isomers and chiral molecules from each other.
Raman spectroscopy became commercially available after invention of lasers in late 1960. A laser beam having a narrow line width is used to illuminate the testing chemicals in solid, liquid or gas forms. The narrow line width of the laser beam can eliminate the overlaps of scattering peaks from photons with various wavelengths. The scattered light is collected by a photon detector such as Charge-Coupled Devices (CCD) or CMOS detector, a Raman spectrum is collected. The Raman shift is defined as the wavelength spacing between the scattering light wavelength and incident light wavelength (laser wavelength). The positions of the peaks correspond to the vibration strengths of various molecular bonds, thus provide a spectral fingerprint of the molecules.
Although Raman scattering is a useful analytical tool, it suffers a major drawback: the scattering signal is very weak due to the very small scattering cross section of molecules. Typically, only about 10−8 of the incident photons on the chemicals will undergo Raman scattering. Of course, high power laser and high sensitivity CCD detector can be used to improve the scattering signals but coming with the extra costs, additional hardware, and unexpected sample damage. Because of the weak scattering signals, normal Raman scattering application is relatively broad but still very limited.
Surface-enhancement effect by using a roughened surface was found to boost Raman scattering signal. In Surface-Enhanced Raman Spectroscopy (SERS), the sample surface can be formed by deposition of metallic particles or clusters. The surface-enhanced Raman scattering phenomena can be explained by interaction between photons with localized electromagnetic field enhancement and chemical enhancement. The enhancement by SERS has been observed in different research labs. An Intel team used a porous silicon structure with coatings of noble metals such as silver on the surface. The Intel team demonstrated that the enhancement increases as the porous silicon pore-size decreases. All the experiments including the work from Intel can be repeated by another team, but it is difficult to reproducibly demonstrate the same level of enhancement.
Accordingly, there is a need to develop well-controlled nano-surface structures at low cost in order to realize commercialization of SERS for various applications ranging from cargo inspection, food inspection, environment monitoring, disease diagnosis, to forensic and homeland security. There is a need to improve the performance of SERS devices and processing techniques for making the same.